1. Field of the Invention
This invention relates to a cleaning technique for a semiconductor substrate, and particularly to a cleaning method for a semiconductor substrate with acid cleaning solution.
2. Prior Art
A high integration technique for semiconductor devices have been increasingly developed, and upon following this increasing development of the high integration, it has been required that semiconductor substrates should be cleaned with high purification and also impurities contained in fine groove portions on the semiconductor substrates should be selectively removed.
A cleaning method using cleaning solution which contains components of HCl/H.sub.2 O.sub.2 /H.sub.2 O=1:1:5 has been known as a conventional method of cleaning semiconductor substrates. Further, Japanese Laid-open Patent Application No. 3-208899 discloses a wet cleaning method using dilute acid which is obtained by properly mixing HCl of 0.1% or more, H.sub.2 O.sub.2 of about 0.05 to 10 wt % and HF of 0.05 to 10 wt %, and a dry cleaning method using light and Cl.sub.2 gas. The wet cleaning method is classified into two types, a strong acid cleaning method and a dilute acid cleaning method.
In the strong acid cleaning method (below pH 1), metallic impurities on a semiconductor substrate is disolved and removed with HCl/H.sub.2 O.sub.2 water-solution having strong oxidizing power. In the dilute acid cleaning method (about pH 3), metallic impurities 3 are removed with dilute hydrofluoric acid simultaneously with removal of naturally-oxidized (natural oxide) films 2 which are naturally formed on the surface of the substrate, and metallic impurities on the semiconductor substrate 1 are oxidized and solved with dilute HCl/H.sub.2 O.sub.2 to be removed. That is, various metallic impurities are effectively removed through a competitive reaction as shown in FIG. 1.
On the other hand, in the dry cleaning method, metallic impurities 3 on the semiconductor substrate 1 are coupled with chlorine radicals which are produced from Cl.sub.2 gas at all times, thereby removing the metallic impurities from the semiconductor substrate 1. The following reaction occurs on the basis of an action between Cl.sub.2 and light: ##STR1##
According to this method, metal trapped in fine groove portions on the semiconductor substrate can be removed because gas is used.
However, the conventional cleaning method as described above still has the following problems. That is, in the strong acid cleaning method of the wet cleaning method, the metallic impurities on the surface of the semiconductor substrate can be easily oxidized and solved. However, the cleaning solution hardly penetrates into fine groove portions because it has high viscosity, and even if the cleaning solution penetrates into the fine groove portions, the metallic impurities are hardly discharged from the surface of the semiconductor substrate into the cleaning solution. Therefore, the metallic impurities may re-precipitate on the surface of the semiconductor substrate in a subsequent rinse process with pure water.
In the dilute acid cleaning method using dilute HCl/H.sub.2 O.sub.2 /FH cleaning solution, the cleaning solution has low acidity, and thus the metallic impurities which have once been eluted from the surface of the semiconductor into the cleaning solution may adhere to the surface of the semiconductor substrate again. In addition, the cleaning solution has insufficient metal-ionizing power because it has weak oxidizing power, and thus it has low efficiency to solve the metal. Therefore, in consideration of the above two disadvantages, this method is ineffective to remove the metal.
In the dry cleaning method using light and Cl.sub.2, the metallic impurities in the fine groove portions are more easily removed than in the wet cleaning method, however, adherence of particles to the surface of the semiconductor substrate is unavoidable because the dry cleaning process is used.
Particularly, the recent progress of designing semiconductor devices in a fine structure has increasingly intensified the effect of metallic impurities on characteristics of the semiconductor devices and the manufacturing yield thereof. When the metallic impurities adhere to the surface of the semiconductor, an insulation break-down voltage of a silicon oxide film is remarkably lowered and increase of p-n junction leak current is induced, so that the characteristics of the semiconductor devices are deteriorated and the yield is lowered.